Hydrolytically degradable polymers and hydrogels made therefrom

ABSTRACT

A water soluble polymer is provided having two or more oligomers linked to each other by hydrolytically degradable carbonate linkages. The polymer can be hydrolytically degraded into oligomers (e.g., oligomers of ethylene oxide) under physiological conditions. The polymer can be conjugated to biologically active agents such as proteins or peptides to impart improved water solubility, reduced immunogenicity, reduced rate of renal clearance, and increased stability. The polymer is useful in making hydrolytically degradable hydrogels which can be used in drug delivery and related biomedical applications. On example of the polymer is a poly (ether carbonate) of the formula X—O—[(—CH 2 CH 2 -0-) n —CO 2 —] n —(CH 2 CH 2 )) n -y where X and Y are independently H, alkyl, alkenyl, aryl, or a reactive moiety, and at least one of X and Y is a reactive moiety.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.10/076,018, filed Feb. 14, 2002, which is a continuation application ofU.S. application Ser. No. 09/459,312, filed Dec. 10, 1999, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention generally relates to water soluble, hydrolyticallydegradable, and nonpeptidic polymers, and particularly to polymers andgels therefrom that are suitable for medical devices, drug enhancementand in vivo delivery of biologically active agents.

BACKGROUND OF THE INVENTION

Conjugation of agents having a known or potential therapeutic benefit towater soluble, non-immunogenic polymers can impart to the agentdesirable characteristics including, among others, improved solubility,greater stability, and reduced immunogenicity. It has becomeincreasingly important in the pharmaceutical industry to developconjugates having these characteristics so as to increase the number ofoptions for therapeutic benefit.

An example of a polymer that can be used to develop conjugates withtherapeutic agents is poly(ethylene glycol) (“PEG”). Therapeutic agentsconjugated to PEG are sometimes said to be “PEGylated.” SeveralPEGylated therapeutics have been developed that exhibit enhanced watersolubility, longer circulation lifetimes, and lower immunogenicity ascompared to the unconjugated therapeutic agent. Because of the rapidmotion and heavy hydration of the polymer, PEGs usually are of muchhigher apparent molecular weight than the therapeutics to which they areattached. Thus, they tend to mask the therapeutic agent from the immunesystem and to preclude excretion through kidneys.

The term PEG is commonly used to describe any of a series of polymershaving the general formula HO—(CH₂CH₂—O)_(n)—H, where “n” represents thenumber of ethylene oxide monomers in the polymer. However, the parentpolymer is generally unsuitable for attachment to a therapeutic agent.Hydroxyl groups are relatively unreactive toward groups commonly presenton therapeutic agents and thus PEG normally has to be “activated” byconverting at least one end hydroxyl group into a more reactive form. Itis also usually important to activate the PEG polymer with a terminalgroup that is selective in its reactions. For example, several PEGderivatives have been developed that are more likely to react with aminegroups. Others have been developed that preferentially react with thiolgroups.

Successful PEG derivatives may have to meet a number of requirements,depending on the specific application. For conjugation to proteins, thePEG derivative should usually have a desirable and suitably selectivereactivity at physiologic conditions of temperature, pressure, and pH topreserve the activity of the unconjugated protein. In somecircumstances, it is desirable to cleave the PEG polymer from thetherapeutic agent at some point after the agent is delivered in vivo.

Some PEG derivatives have been used in combination with other polymersto prepare insoluble gels in which drugs can be entrapped or chemicallybound. For example, Sawhney et al., Macromolecules, 26:581 (1993)describes the preparation of block copolymers of PEG with polyglycolideor polylactide blocks at both ends of the PEG chain. The copolymers arethen activated by terminal substitution with acrylate groups, as shownbelow.CH₂═CH—CO—(O—CH₂—CO)_(n)—O-PEG-O—(CO—CH₂—O)_(n)—CO—CH═CH₂In the above formula, the glycolide blocks are the —O—CH₂—CO— units. Theaddition of a methyl group to the methylene gives a lactide block; n canbe multiples of 2. Vinyl polymerization of the acrylate groups producesan insoluble, crosslinked gel with a polyethylene backbone.

The polylactide or polyglycolide segments of the polymer backbone shownabove, which are ester groups, are susceptible to slow hydrolyticbreakdown, with the result that the crosslinked gel undergoes slowdegradation and dissolution. The hydrogel degrades in vivo and canresult in non-PEG components being released into the blood stream, whichcan be undesirable.

It is desirable to develop improved polymers providing additionalchoices for use in drug delivery and other applications.

SUMMARY OF THE INVENTION

This invention provides a water soluble, nonpeptidic polymer having twoor more oligomers linked together by hydrolytically degradable carbonatelinkages. The polymer can be hydrolytically degraded into smalloligomers in an aqueous environment, including in vivo conditions. Thepolymer is easy to prepare and the molecular weight of the oligomersresulting from polymer degradation can be easily controlled, which canbe desirable for some applications. The polymer can be conjugated to abiologically active agent such as a protein or peptide. The polymer canimpart desirable characteristics to the conjugates of improved watersolubility and reduced immunogenicity. The polymer is useful forpreparing insoluble cross linked structures, including hydrogels, thatare hydrolytically degradable into soluble polymers of predeterminedmolecular weight.

The oligomers are alkylene oxide oligomers. Typically, the oligomers areethylene oxide oligomers, and the polymer is a poly(ether carbonate)having the formula of:HO—[(—CH₂CH₂—O)_(n)—CO₂]_(m)—(—CH₂CH₂—O)_(n)Hwhere n is from about 1 to 2,000, normally from 2 to 2,000, and m isfrom about 2 to 200. Since carbonate linkages are hydrolyticallydegradable under mild conditions, the polymer will hydrolyze to produceoligomer fragments of much lower molecular weight than the startingpolymer:HO—[(—CH₂CH₂—O)_(n)—CO₂]_(m)—(—CH₂CH₂—O)_(n)H+(m+1) H₂O→(m+1)HO—(—CH₂CH₂—O—)_(n)—H+mCO₂In addition to providing many of the desirable features of otherpolymers, including poly (ethylene glycol) as described above, this newpolymer can degrade in the body and thus facilitates removal of thepolymer from the body. The degradation products are themselves normallynontoxic small PEGs that typically are rapidly cleared from the body.

The polymer can be prepared in a number of ways. In one embodiment ofthis invention, the poly(ether carbonate) is prepared by polymerizing anactivated oligomer having the formula of:HO—(—CH₂CH₂—O—)_(n)—CO₂-Zwhere n can be from about 2 to 2000 and Z is a reactive leaving groupsuch as N-succinimidyl, 1-benzotriazolyl, or p-nitrophenyl.

The polymer can be prepared by polymerizing ethylene oxide oligomers ofthe formula:HO—(—CH₂CH₂—O—)_(n)—Hwhere n can be from about 2 to 2000 with an activating molecule ofZ-O—CO₂-Z, where Z is as described.

Alternatively, the ethylene oxide oligomerHO—(—CH₂CH₂—O—)_(n)—Hcan be polymerized with a bifunctional ethylene oxide oligomer:Z-OCO₂—(—CH₂CH₂—O—)_(n)—CO₂-Zwhere n and Z are as described above, to form the poly(ether carbonate).

The polymerization reactions may be conducted either in an organicsolvent or in a melt, in the presence of an organic base. Examples ofsuitable solvents include acetonitrile, THF, dimethylformamide,dimethylsulfoxide, benzene, toluene, the xylenes, chloroform, andmethylene chloride. Examples of suitable organic bases includetriethylamine, pyridine, quinoline, 4,4-dimethylaminopyridine andtriethylamine. The polymerization reactions can be conducted at atemperature of from about 37° C. to 100° C., typically from about 45° C.to 100° C., and advantageously from about 70° C. to 90° C.

The polymer of this invention can be modified at one termius with alkylor aryl groups to make one end of the polymer inert. The polymer can beactivated at one or more of its termini to form a terminal reactivemoiety. Thus, a modified or activated poly(ether carbonate) of thisinvention can be represented as:X—O—[(CH₂CH₂—O)_(n)—CO₂]_(m)—(CH₂CH₂—O)_(n)—Ywhere m and n are as defined above, and where X and Y can independentlybe H, alkyl, alkenyl, aryl, and reactive terminal moieties, includingN-succinimidyloxycarbonyl, 1-benzotriazolyloxycarbonyl,p-nitrophenyloxycarbonyl, or others. Alternatively, X and Y can includelinker groups terminating in active groups such as aldehyde N-maleimidylor —S—S-ortho-pyridyl. A wide variety of activating groups and linkerscan be used.

The activated polymer of this invention can be reacted with an activegroup on a biologically active agent, such as a protein or peptide, toform a conjugate. For example, N-succinimidyloxy, 1-benzotriazolyloxy,and p-nitrophenyloxy are leaving groups suitable for the formation of acarbamate linkage between the polymer and a biologically active agenthaving an amino group. Thus proteins, peptides, amino drugs, or aminocarbohydrates can be linked to such activated polymers. For example,when X is H, and Y is N-succinimidyloxycarbonyl, a conjugate can beformed of the following formula:HO—[(CH₂CH₂—O)_(n)—CO₂]_(m)—(CH₂CH₂—O)_(n)—CONH-Protein

When a protein to be conjugated has an accessible thiol group, thepolymer of this invention can be activated to contain a terminalreactive moiety that is reactive with thiol, including, for example,iodoacetamide, vinylsulfone, maleimide, or S—S-ortho-pyridyl, whichmoiety is then reacted with the thiol group to form a thiolsite-specific conjugate of the protein.

When the polymer of this invention is activated at two termini, it canbe used as a crosslinking agent to crosslink a multifunctional moleculeto form a hydrolytically degradable hydrogel. Examples ofmultifunctional molecules suitable as “backbones” in formation ofhydrogels include proteins such as collagen, aminocarbohydrates such aschitosan, polyamines such as polylysine and poly(vinylamine), andmulti-armed or branched poly(ethylene glycol) amines. The hydrogels ofthis invention are useful in many biomedical applications such as drugdelivery, surgical adhesion prevention, wound and scar healing,bioadhesives and surgical implants.

In another embodiment, the polymer of this invention can be activated byattachment of terminal vinyl groups. This activated polymer can beself-polymerized in the presence of a conventional vinyl polymerizationcatalyst to form a hydrolytically degradable hydrogel.

Thus, this invention provides a versatile polymer that is especiallysuited for conjugating to a biologically active agent and for forming ahydrogel. The polymer is easy to prepare and can be synthesized in largequantities. The polymer can be formed in a single reaction with multipledegradable carbonate linkages in the backbone. The conjugate andhydrogel of this invention can be degraded under physiologicalconditions. The degradation generates oligomers of predeterminedmolecular weight that can be easily cleared from the body.

The foregoing and other advantages and features of the invention, andthe manner in which the same are accomplished, will become more readilyapparent upon consideration of the following detailed description of theinvention taken in conjunction with the accompanying examples, whichillustrate preferred and exemplary embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described more fully hereinafter withreference to the accompanying examples, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

As used herein, the term “polymer” means a molecule, formed by thechemical union of two or more oligomer units. The chemical units arenormally linked together by covalent linkages. The two or more combiningunits in a polymer can be all the same, in which case the polymer isreferred to as a homopolymer. They can be also be different and, thus,the polymer will be a combination of the different units. These polymersare referred to as copolymers.

The term “oligomer” is used herein to mean a molecule, typically anorganic molecule, which in itself is formed by the chemical union of twoor more monomer units. The monomer units of an oligomer may be differentor all the same. An oligomer is capable of reacting with anotheroligomer which is same or different, in a polymerization reaction toform a polymer. As used herein, the term oligomer by no means limits thesize of the molecule or the number of combining units or monomers in theoligomer. Rather, “oligomer” is used to indicate a unit for forming apolymer of the invention. The structure of an oligomer in a polymer maybe somewhat different in chemical structure from the oligomer prior topolymerization because of the polymerization reaction and the formationof covalent linkages.

The term “carbonate linkage” is used herein to mean a linkage thatincludes the group —O—CO₂—. It is to be understood that a carbonatelinkage is distinct from a carboxylate linkage which typically has astructure of R—CO₂— (where n is at least 1 and R=alkyl or aryl) and hasdifferent chemical and physical properties.

The terms “group,” and “moiety,” are all used herein interchangeably torefer to a distinct, definable portion or unit of a molecule. Sometimes,the structure of a group or moiety may include another smaller group ormoiety. For example, a functional group of —O—CO₂-Z includes Z which maybe a reactive group or moiety including N-succinimidyl or1-benzotriazolyl.

The terms “active,” “reactive,” and “functional” are used hereininterchangeably to mean that a molecule or a group or moiety is reactivewith other molecules or groups or moieties of molecules.

The term “biologically active agent” when used herein means anysubstance that can impact any physical or biochemical properties of abiological organism including, but not limited to, viruses, bacteria,fungi, plants, animals and humans. In particular, as used herein, abiologically active agent can be any substance intended for thediagnosis, cure, mitigation, treatment, or prevention of a disease inhumans or other animals, or to otherwise enhance the physical or mentalwell being of humans or animals. Examples of biologically active agentsinclude, but are not limited to, organic and inorganic compounds,proteins, peptides, lipids, polysaccharides, nucleotides, DNAs, RNAs,other polymers, and derivatives thereof. Examples of biologically activeagents include antibiotics, fungicides, anti-viral agents,anti-inflammatory agents, anti-tumor agents, cardiovascular agents,anti-anxiety agents, hormones, growth factors, steroidal agents, and thelike. Biologically active agents include microorganisms such as bacteriaand yeast cells, viral particles, plant or animal or human cells ortissues, and the like, in their native or modified forms.

The oligomers used in the practice of the invention should be selectedso that they and the degradation products of the polymer of thisinvention are water soluble and can easily be excreted from animalbodies under natural physiological conditions. They should be non-toxic,or at least of acceptable low toxicity, and should not cause asubstantial adverse effect in human or animal bodies.

Many different types of alkylene oxide oligomers are useful in formingthe polymer of this invention. In its simplest form, an alkylene oxide“homo-oligomer” is used having the formula of HO—(—CHRCH₂—O)_(n)—H,where R is H or an alkyl, including methyl. Suitable oligomers alsoinclude alkylene oxide “co-oligomers,” which are composed of differentalkylene oxide monomers in which R is varied independently along thechain. An example of a co-oligomer is a structure in which two differentR groups are present in a block such asHO—(—CHR₁CH₂—O—)_(n)—(—CHR₂CH₂—O—)_(n)—H, where n and m can be variedindependently. An example of this type of block structure is the case inwhich R₁ is hydrogen and R₂ is methyl.

Block oligomers can exhibit surface activity. Degradable surfactants areuseful in drug delivery systems and can be used to form micelles andthermally reversible gels. Random oligomers in which R₁ and R₂ arerandomly placed along the oligomer chain are also useful. The oligomerscan be branched, as in the formulaR₃[O—(—CHR₁CH₂—O—)_(n)-]_(m)Hwhere R₃ is a core branching moiety, including glycerol orpentaerythritol, R₁ is alkyl, n can range from 2 to hundreds, and m isat least 3.

A suitable oligomer can be formed from about 2 to 2000 monomers.However, the size of the oligomers used can vary so long as the aboverequirements are met. Normally an oligomer has from about 5 to 500monomers. Advantageously, an oligomer has from about 10 to 50 monomers.An oligomer can be prepared by polymerizing or co-polymerizing monomers,and the size or molecular weight of the oligomer can be controlled bylimiting the extent of this polymerization reaction.

As noted above, the oligomers contained in the polymer of this inventioncan be the same or different types of oligomers, or oligomers of thesame type but different sizes. Therefore, the polymer of this inventioncan be either a homopolymer or heteropolymer.

The polymer of this invention typically has one single linear polymerbackbone with two termini. However, branched polymers and star polymersare also contemplated that have two or more linear polymers, at leastone of which is the linear polymer backbone of this invention,covalently linked to a central branching core.

The polymer of this invention can have an inert terminal moiety,typically H, alkyl, and aryl. The alkyl and aryl groups can besubstituted or unsubstituted, and normally are methyl, ethyl, phenyl,etc. The polymer can also have one or more reactive moieties capable ofreacting with a moiety in another molecule, such as an amino group or athiol group on a protein. Examples of such reactive moieties include,but are not limited to, acryloyl, alkenyl, tresyl,N-succinimidyloxycarbonyl, 1-benzotriazolyloxycarbonyl,p-nitrophenyloxycarbonyl, N-maleimidyl, aldehydes, acetals,1-imidazolylcarbonyl, vinylsulfone, iodoacetamide, o-pyridyldithiyl, andthe like.

In one embodiment of this invention, the hydrolytically degradablepolymer is a poly(ether carbonate) composed of two or more alkyleneoxide oligomers and/or alkylene oxide monomers covalently linkedtogether by carbonate linkages. For example, the poly(ether carbonate)can have the formula ofX—O—{[(R₁—O)_(a)—CO₂—]_(h)—[(R₂—O)_(b)—CO₂—]_(i)}_(m)—{[(R₃—O)_(c)—CO₂—]_(j)—[(R₄—O)_(d)—CO₂—]_(k)}—(R₅—O)_(e)—Y

wherein

R₁, R₂, R₃, R₄, and R₅ are alkyl groups which may be linear, branched,substituted or unsubstituted, and they can be same or different;typically R₁, R₂, R₃, R₄, and R₅ are ethyl;

a, b, c, d, e each is an integer of from 1 to about 2000, normally fromabout 5 to 500, and advantageously from about 10 to 50;

h, i, j, and k each is an integer of from 0 to about 100, and the sum ofh, i, j, and k is from about 2 to 200, normally from about 5 to 100, andadvantageously from about 10 to 50; and

each of X and Y is hydrogen, alkyl, alkenyl, aryl, or reactive moietiesas described above, and X and Y can be same or different.

In a preferred embodiment, the poly(ether carbonate) of this inventionhas the formula of:X—O—[(—CH₂CH₂—O—)_(n)—CO₂—]_(m)—(CH₂CH₂O)_(n)—Ywherein n is an integer of from about 2 to 2000, normally from about 5to 500, and advantageously from about 10 to 50; m is an integer of fromabout 2 to 100, typically from about 5 to 100, and advantageously fromabout 10 to 50, and wherein X and Y can be same or different and eachindependently is H, alkyl, alkenyl, aryl, or a reactive moiety,acryloyl, tresyl, N-succinimidyloxycarbonyl,1-benzotriazolyloxycarbonyl, p-nitrophenyloxycarbonyl, N-maleimidyl,aldehydes, acetals, 1-imidazolylcarbonyl, vinylsulfone, iodoacetamide,and o-pyridyldithiyl.

This specific form of poly(ether carbonate) contains repeating ethyleneoxide oligomers linked by carbonate linkages that can be hydrolyticallycleaved. Such hydrolytic cleavage leads to ethylene oxide oligomers andcarbon dioxide. Therefore, the poly(ether carbonate) differssubstantially from poly(ethylene glycol) or PEG in having multipledegradable backbone carbonate linkages that allow the polymer to bebroken down into many smaller oligomers. Since the rate of degradationof the polymer is proportional to the number of degradable carbonatelinkages present, and since the size and number of the oligomers can bepredetermined, substantial control over both degradation rate and thesize of the degradation products is thus possible.

To prepare polymers of the invention, in one example, one or moreoligomers as described above are provided each having a hydroxyl groupat one terminus and a functional group of —O—CO₂-Z at another terminus.The oligomers are then polymerized or co-polymerized in a condensationpolymerization reaction under conditions sufficient to form ahydrolytically degradable polymer.

The functional group —O—CO₂-Z is capable of reacting with a hydroxylgroup to form a carbonate linkage. Typically, Z can be any reactiveleaving groups so long as the functional group can react with a hydroxylgroup to form a carbonate linkage. Examples of suitable leaving groupsinclude N-succinimidyl, 1-benzotriazolyl, and p-nitrophenyl. Methods forpreparing an oligomer having a functional group —O—CO₂-Z as describedabove are well known in the art, and are disclosed in U.S. Pat. Nos.5,650,234, 5,281,698 and 5,468,478; Veronese, et al., Appl. Biochem.Biotech., 11:141 (1985); and Sartore et al., Appl. Biochem. Biotech.,27:45 (1991), all of which are incorporated herein by reference.

As discussed above, one or more types of oligomers can be polymerizedtogether. In addition, if desired, monomers having a hydroxyl terminusand a functional group —O—CO₂-Z at another terminus, can also beincluded in the polymerization mixture.

Thus, to give an example, the poly(ether carbonate)HO—[(—CH₂CH₂—O—)_(n)—CO₂—]_(m)—(CH₂CH₂O)_(n)—Has described above can be prepared in accordance with this method by thefollowing steps. First, an ethylene oxide oligomer is provided having aformula of HO—(—CH₂CH₂—O—)_(n)—O—CO₂-Z, where Z is a reactive leavinggroup such as N-succinimidyl, 1-benzotriazolyl, and p-nitrophenyl. Thisoligomer is then polymerized under controlled conditions to provide theabove poly(ether carbonate).

In another example of this invention, a first oligomer is provided,which is a bifunctional oligomer having a first functional group of—O—CO₂—W at one terminus and, at another terminus, a second functionalgroup of —O—CO₂-Z. Both functional groups are capable of reacting with ahydroxyl group to form a carbonate linkage. Z and W are reactive leavinggroups, and can be any leaving groups known in the art so long as thefunctional groups containing them, as stated above, are capable ofreacting with a hydroxyl group to form a carbonate linkage. Z and W canbe same or different. The preferred Z and W are N-succinimidyl,1-benzotriazolyl, and p-nitrophenyl. Two or more different bifunctionaloligomers can also be used in the same polymerization reaction.

Methods for preparing such bifunctional oligomers are similar to thosefor making the monofunctional oligomers described above. Preferably, Zand W are the same, and the bifunctional molecule Z-oligomer-Z can beprovided by activating an oligomer having two hydroxyl terminal groups,with an activating molecule having the formula of Z-O—CO₂-Z. Suitableexamples of the activating molecule include, disuccinimidylcarbonate,bis(1-benzotriazolyl) carbonate and bis(p-nitrophenyl) carbonate. See,e.g., U.S. Pat. No. 5,281,698; U.S. Pat. No. 5,650,234; Veronese, etal., Appl. Biochem. Biotech., 11:141 (1985); and Sartore et al., Appl.Biochem. Biotech., 27:45 (1991), all of which are incorporated herein byreference.

In addition to the first oligomer that is bifunctional, a secondoligomer is also provided having two terminal hydroxyl groups. Thissecond oligomer is then polymerized with the first oligomer to form thepolymer of this invention.

Two or more types of bifunctional oligomers can be used in the samepolymerization reaction. In addition, two or more types of oligomershaving two terminal hydroxyl groups can also be used in a polymerizationreaction. As will be apparent to a skilled artisan, when only one typeof bifunctional oligomer and one type of dihydroxyl oligomer are used,and when the two oligomers are the same except for the terminal groups,the polymer formed therefrom will be a homopolymer having a single typeof repeating unit or oligomer linked with hydrolytically degradablelinkages. Otherwise, a heteropolymer or block polymer or terpolymer willbe made containing different types of oligomers in the polymer backbone.

To give an example of the method of this embodiment, the poly(ethercarbonate)HO—[(—CH₂CH₂—O—)_(n)—CO₂—]_(m)—(CH₂CH₂O)_(n)—Hcan be prepared by co-polymerizing a first ethylene oxide oligomerhaving a formula ofZ-O₂C—O—(—CH₂CH₂—O—)_(n)—CO₂-Zand a second ethylene oxide oligomer HO—(—CH₂CH₂—O—)_(n)—OH, undersuitable polymerization conditions. Z is preferably N-succinimidyl,1-benzotriazolyl, or p-nitrophenyl. The oligomerZ-O₂C—O—(—CH₂CH₂—O—)_(n)—CO₂-Zcan be prepared by reacting Z-O—CO₂-Z with HO—(—CH₂CH₂—O—)_(n)—OH, underthe conditions disclosed in U.S. Pat. No. 5,281,698.

In yet another method, one or more oligomers having two hydroxylterminal groups are polymerized directly with an activating moleculehaving the formula of Z-O—CO₂-Z to form the hydrolytically degradablepolymer. Thus, for example, the poly(ether carbonate)HO—[(—CH₂CH₂—O—)_(n)—CO₂—]_(m)—(CH₂CH₂O)_(n)—Hcan also be prepared by co-polymerizing an activating molecule Z-O—CO₂-Zand the oligomer HO—(—CH₂CH₂—O—)_(n)—OH, under conditions sufficient toform the poly(ether carbonate).

Again, in this method, either one oligomer is used to generate ahomopolymer, or two or more different oligomers can be used in the samepolymerization reaction to produce a heteropolymer, or block polymer orterpolymer.

The polymerization reactions are conducted under conditions sufficientto form the hydrolytically degradable polymer of this invention. Thepolymerization reaction in each of the above-described methods is acondensation reaction. Many different known reaction conditions can beused. Typically, a catalyst is included in the polymerization reactionmixture. Examples of suitable catalysts are organic bases, includingtriethylamine, pyridine, quinoline, and 4,4-dimethylaminopyridine. Aminebases such as 4,4-dimethylaminopyridine and triethylamine are preferred.

The polymerization can be conducted in either melt or solvent. Suitablesolvents include, but are not limited to, acetonitrile, THF,dimethylformamide, dimethylsulfoxide, benzene, toluene, xylenes,chloroform, and methylene chloride. The polymerization reaction rate andthe extent of polymerization, which determines the average moleculeweight of the final hydrolytically degradable polymer product can bepartly controlled by the reaction temperature and the reaction time.Suitable reaction temperature can vary from about 0° C. to 100° C.Higher reaction temperatures lead to greater reaction speed. Preferably,the polymerization reaction is conducted at a temperature of from about37° C. to 100° C., typically from about 45° C. to 100° C. andadvantageously from about 70° C. to 90° C. When the reaction isconducted in a melt, the temperature needs be maintained at a certainminimum temperature in order to keep the reaction mixture at a meltstate.

In the above described three embodiments of the method of thisinvention, the polymerization reactions would be predicted to lead topolymers with an activated carbonate terminal group. In practice,however, NMR analysis of the polymer products indicates that theterminal groups of the hydrolytically degradable polymer preparedtherefrom often are hydroxyl groups. While not wishing to be bound byany theory, it is believed that this is caused by reaction with a smallamount of water present as an impurity in the reaction. Any small amountof remaining terminally activated carbonate may be removed by hydrolyisin water for a short period or near a neutral pH. The terminalactivating groups are much more sensitive to water than are thedegradable carbonate linkages.

The polymer of this invention can optionally be activated at one or alltermini, thus providing an activated polymer capable of being covalentlylinked to another molecule, including, for example, a protein, to form aconjugate. The polymer can also be capped at one terminus by an inertgroup and at another terminus by a reactive moiety.

The polymer of this invention can be activated at its terminus to form aterminal reactive moiety by methods well known to those familiar withthe art of organic or polymer chemistry. The well established methods inthe broad field of poly(ethylene glycol) chemistry are generally useful,and such methods should be apparent to a skilled artisan. The polymercan be activated at one terminus, or all termini, in which case, thereactive moieties at different termini can be same or different.

For example, the polymer may be activated to form a terminal moiety ofN-succinimidyl carbonate (see e.g., U.S. Pat. Nos. 5,281,698,5,468,478), amine (see, e.g., Buckmann et al. Makromol. Chem. 182:1379(1981), Zaplipsky et al. Eur. Polym. J. 19:1177 (1983)), hydrazide (See,e.g., Andresz et al. Makromol. Chem. 179:301 (1978)), succinimidylpropionate and succinimidyl butanoate (see, e.g., Olson et al. inPoly(ethylene glycol) Chemistry & Biological Applications, pp 170-181,Harris & Zaplipsky Eds., ACS, Washington, D.C., 1997; see also U.S. Pat.No. 5,672,662), succinimidyl succinate (See, e.g., Abuchowski et al.Cancer Biochem. Biophys. 7:175 (1984) and Joppich et al. Macrolol. Chem.180:1381 (1979), succinimidyl ester (see, e.g., U.S. Pat. No.4,670,417), benzotriazole carbonate (see, e.g., U.S. Pat. No.5,650,234), glycidyl ether (see, e.g., Pitha et al. Eur. J. Biochem.94:11 (1979), Elling et al., Biotech. Appl. Biochem. 13:354 (1991),oxycarbonylimidazole (see, e.g., Beauchamp, et al., Anal. Biochem.131:25 (1983), Tondelli et al. J. Controlled Release 1:251 (1985)),p-nitrophenyl carbonate (see, e.g., Veronese, et al., Appl. Biochem.Biotech., 11:141 (1985); and Sartore et al., Appl. Biochem. Biotech.,27:45 (1991)), aldehyde (see, e.g., Harris et al. J. Polym. Sci. Chem.Ed. 22:341 (1984), U.S. Pat. No. 5,824,784, U.S. Pat. No. 5,252,714),maleimide (see, e.g., Goodson et al. Bio/Technology 8:343 (1990), Romaniet al. in Chemistry of Peptides and Proteins 2:29 (1984)), and Kogan,Synthetic Comm. 22:2417 (1992)), orthopyridyl-disulfide (see, e.g.,Woghiren, et al. Bioconj. Chem. 4:314 (1993)), acrylol (see, e.g.,Sawhney et al., Macromolecules, 26:581 (1993)), vinylsulfone (see, e.g.,U.S. Pat. No. 5,900,461). In addition, two molecules of the polymer ofthis invention can also be linked to the amino acid lysine to form adi-substituted lysine, which can then be further activated withN-hydroxysuccinimide to form an active of N-succinimidyl moiety (see,e.g., U.S. Pat. No. 5,932,462). All of the above references areincorporated herein by reference.

To give an example, the polymer of this invention may be activated toform a terminal reactive moiety of N-succinimidyl carbonate or1-benzotriazolyl carbonate by reacting the polymer withdi-N-succinimidyl carbonate or di-1-benzotriazolyl carbonaterespectively. To give another example, terminal reactive moieties suchas N-maleimidyl and o-pyridyldithiyl may be prepared by reacting thepolymer with activated carbonates connected to N-maleimidyl oro-pyridyldithiyl by linker groups. Terminal aldehyde and acetal moietiescan be attached by linking groups. Terminal acid groups can be attachedby reaction of the above active carbonates with amino acids or otheracid linkers. These acids can then be activated by formation of activeesters such as succinimidyl active esters.

The polymer of this invention, activated or not, as prepared by theabove methods, can be easily purified from the reaction mixture. Manymethods known in the art can be used. A preferred method for purifyingthe polymer and its derivatives is by precipitation from a solvent inwhich they are essentially insoluble while the reactants are soluble.Suitable solvents include ethyl ether or isopropanol. As is apparent toa skilled artisan, other methods such as ion exchange, size exclusion,silica gel, and reverse phase chromatography can also be useful.

In accordance with another aspect of this invention, the activatedpolymer is used to form a conjugate with a biologically active agent.The conjugate comprises the polymer of this invention covalently linkedto a biologically active agent. Because the polymer of this invention ishydrolytically degradable in vivo and can be cleaved at the carbonatelinkages, thus forming smaller oligomers, the conjugate is especiallyuseful for controlled delivery of the bound agent into animal bodies.Since the polymer in the conjugate is susceptible to breakdown intosmaller oligomers, the polymer typically is excreted from animal bodiesfaster than a polymer of similar size that is not degradable. Thus,potential adverse effects that may be caused by a large polymer's longperiod of stay in the body is avoided. Therefore, conjugation of thepolymer of this invention to a biologically active agent can provide fora sufficient circulation period for the agent while minimizing anyadverse effects of the polymer.

In the conjugates of this invention, the linkage between thebiologically active agent and the polymer of this invention can bestable or hydrolytically degradable. When it is degradable,substantially all of the polymer can be cleaved off the biologicallyactive agent under physiological conditions, releasing the agentsubstantially in its native form inside the body. Methods for forming ahydrolytically degradable linkage between a biologically active agentand a water soluble polymer are well known in the art and should beapparent to a skilled artisan. For example, ester linkages formed by thereaction of PEG carboxylic acids or activated PEG carboxylic acids withalcohol groups on a biologically active agent generally hydrolyze underphysiological conditions to release the agent. Other hydrolyticallydegradable linkages include carbonate linkages; imine linkages resultedfrom reaction of an amine and an aldehyde (see, e.g., Ouchi et al.,Polymer Preprints, 38(1):582-3 (1997), which is incorporated herein byreference.); phosphate ester linkages formed by reacting an alcohol witha phosphate group; hydrozone linkages which are reaction product of ahydrazide and an aldehyde; acetal linkages that are the reaction productof an aldehyde and an alcohol; orthoester linkages that are the reactionproduct of a formate and an alcohol; peptide linkages formed by an aminegroup, e.g., at an end of a polymer such as PEG, and a carboxyl group ofa peptide; and oligonucleotide linkages formed by a phosphoramiditegroup, e.g., at the end of a polymer, and a 5′ hydroxyl group anoligonucleotide.

Methods for conjugating the polymer of this invention to a biologicallyactive agent should be apparent based on the above discussion.Typically, the polymer of this invention must be activated to form theactivated polymer of this invention as described above, having at leastone terminal reactive moiety. The terminal reactive moiety may varydepending on the reactivity of a target moiety on the biologicallyactive agent to be conjugated. Examples of reactive groups on proteinsare thiols and amines, while on small molecule drugs, amines, alcohols,thiols, and carboxylic acids are common reactive gorups. The conjugateis then formed by reacting the terminal reactive moiety of the activatedpolymer with the target moiety on the biologically active agent. Suchmethods are well known in the art, and are discussed in the patents andpublications referred to above in the context of forming terminalreactive moieties.

In a preferred embodiment, the polymer of this invention used forforming a conjugate is a poly(ether carbonate) composed of alkyleneoxide oligomers, more preferably ethylene oxide oligomers, linkedtogether by carbonate linkages. Ethylene oxide oligomers arepoly(ethylene glycol)s with a predetermined molecular weight, typicallyfrom about 88 to about 8000, preferably from about 88 to about 2000.Thus, in this embodiment of the invention, the polymer behaves in asimilar manner as polyethylene glycol. However, when delivered in vivo,the polymer in the conjugate will break down into a number of smalleroligomer fragments. If the linkage between the polymer and thebiologically active agent is stable, then, after degradation, oneoligomer is linked to the agent.

In accordance with another aspect of this invention, a hydrolyticallydegradable hydrogel and method of making thereof are also provided. Asis known in the art, a hydrogel typically is a polymeric network formedby crosslinking one or more multifunctional backbone molecules orpolymers. The resulting polymeric network is hydrophilic and swells inan aqueous environment thus forming a gel-like material, i.e., hydrogel.Hydrogels are useful for drug delivery as they can be implanted orinjected into animal bodies. Typically a hydrogel comprises a backbonebonded to a crosslinking agent.

In accordance with this invention, the polymer of this invention is usedas the crosslinking agent in the hydrogel. The polymer must be activatedso that it has at least two terminal reactive moieties that are capableof reacting with multiple moieties on the backbone to form covalentlinkages.

Alternatively, two or more types of activated polymer are used ascrosslinking agents. Each activated polymer has one terminal reactivemoiety capable of reacting with a moiety on the backbone, and anotherterminal reactive moiety capable of reacting with the correspondingterminal reactive moiety on the other type of activated polymer. Anexample of this other moiety is, for example, a vinyl-containing groupsuch as an acrylate group that can participate in chain polymerizationamong the different types of activated polymers. When the polymer ofthis invention is activated so that it has two terminal vinyl groups,the polymer itself may act as both crosslinking agent and backbone, andself-polymerize into a hydrolytically degradable hydrogel through achain polymerization reaction.

The backbone of the hydrogel is a nontoxic biocompatible macromoleculeor small molecule, having at least two or preferably more active groupsavailable to react with the terminal reactive moieties of thecrosslinking agent to form covalent linkages. By “biocompatible” it isintended that the molecule used as backbone would not substantiallyadversely affect the body and tissue of the living subject into whichthe hydrogel is to be implanted or injected. More particularly, thematerial does not substantially adversely affect the growth and anyother desired characteristics of the tissue cells surrounding theimplanted hydrogel. It is also intended that the material used does notcause any substantially medically undesirable effect in any other partsof the living subject. In addition, if the molecule is degradable insidethe body, the degradation products should also be substantiallybiocompatible as defined above. Generally, the methods for testing thebiocompatibility of a material well known in the art.

Examples of suitable backbones include, but are not limited to,proteins, modified proteins such as glycoproteins, phosphorylatedproteins, acylated proteins, and chemically modified proteins, peptides,aminocarbohydrates, glycosaminoglycans, aminolipids, polyols,polythiols, polycarboxylic acids, polyamines such as dilysine,poly(vinylamine) and polylysine, poly(ethylene glycol) amines,pharmaceutical agents having at least two active groups, etc. Specificexamples of the backbone include, but are not limited to, branched PEGamines, fibrin, fibrinogen, thrombin, albumins, globulins, collagens,fibronectin, chitosan, and the like. In addition, the backbone may alsobe microorganisms such as viral particles, bacterial or yeast cells,animal or human cells or tissues.

The activated polymer of this invention used as a crosslinking agent canbe in a linear, branched or star form. In branched or star forms, threeor more linear polymers are covalently linked, at one terminus, to acentral, branched core moiety. The central branch core moiety can bederived from the amino acid lysine, or polyols such as glycerol,pentaerythritol and sorbitol. Branched PEGs are known in the art. Thesebranched PEGs can be incorporated as components of the poly(ethercarbonate)s of this invention.

As will be apparent, because of the carbonate linkages incorporated inthe crosslinking agent, the hydrogel of this invention is hydrolyticallydegradable. In addition, the linkages between the backbones and thecrosslinking agents formed from the crosslinking reactions can also bemade hydrolyzable, by methods described in the context of the conjugateof this invention. Thus, the hydrogel of this invention can graduallybreak down or degrade in the body as a result of the hydrolysis of thehydrolytically degradable linkages.

Therefore, the hydrogel of this invention is suitable as a biomedicalmaterial and a carrier for the delivery of biologically active agents.For example, the hydrogel can carry therapeutic drugs and can beimplanted or injected in the target area of the body. The hydrogel mayalso carry other agents such as nutrients or labeling agents for imaginganalysis. A hydrogel containing a biologically active agent is termedherein as “a delivery system”.

In the various applications of the hydrogel of this invention, thebiologically active agents to be delivered can be used as the backbone,or part of the backbone of the hydrogel. Alternatively, biologicallyactive agents can be “hinged” to the hydrogel through a polymer of thisinvention or a linker molecule with one terminus of the polymer or thelinker linked to the biologically active agent, and the other beingconnected through a covalent linkage to the hydrogel. In addition,biologically active agents or other substances to be delivered can alsobe loaded into the hydrogel during the formation of the hydrogel, orafterwards by, for example, diffusion into the matrix of the hydrogelwithout being covalently bonded to the hydrogel structure.

Because the crosslinking agents (i.e., the polymers of this invention)in the hydrogel are water soluble, the hydrogel can be substantiallywater swellable. The degradation or breakdown of the hydrogel in thebody is gradual in nature and subject to control because of thehydrolytically degradable carbonate linkages in the crosslinking agents.Thus, the hydrogels of the invention are particularly useful forsustained release of a biologically active agent or other substance inthe body. The hydrogels have potential utility for adhesion prevention,bioadhesives, surgical sealants, and related surgical applications.

The present invention is further illustrated in the following exampleswhich are given to illustrate the invention, but should not beconsidered in limitation of the invention.

EXAMPLE 1

Synthesis of HO-PEC_(14KDa)—OH from HO—(—CH₂CH—O—)_(n)CO₂Bt(Bt=1-benzotriazolyl) (n_(ave)=23)

HO—(—CH₂CH₂—O—)_(n)CO₂Bt (n_(ave)=23) (5 g) and 1.2 g ofdimethylaminopyridine were dissolved in 10 ml of anhydrous acetonitrile.The reaction was stirred at 70° C. for 22 hours. The product wasprecipitated with 200 ml of isopropanol, stirred for one-half hour, thencollected by vacuum filtration. The product was washed with isopropanol(100 ml×2), and dried in vacuo overnight. Yield: 4.1 g (82%). Theresulting polymer was 99% pure by gpc analysis and had a molecularweight of 13,800 Da with a polydispersity of 1.47.

EXAMPLE 2

Synthesis of HO-PEC_(7K)—OH from Bt-O₂C—(—O—CH₂CH₂—)_(n)—OCO₂-Bt andHO—(—CH₂CH₂—O—)_(n)—H (n_(ave)=23)

1. Preparation of Bt-O₂C—(O—CH₂CH₂—)_(n)—OCO₂—Bt (n_(ave)=23)

HO—(—CH₂CH₂—O—)_(n)—H (n_(ave)=23) (15 g, Aldrich) was dissolved in 100ml of anhydrous acetonitrile, and solvent was removed by distillation.The residue was dissolved in 100 ml of anhydrous acetonitrile, and 13.3g (1.5 equivalent) of di(1-benzotriazolyl carbonate) and 3.1 ml (1.3equiv) of anhydrous pyridine were added. The reaction was stirred atroom temperature under Ar overnight. The solvent was evaporated todryness under reduced pressure. The product was precipitated by additionof 300 ml of cold diethyl ether. The precipitate was stirred under Arfor 30 min., collected by vacuum filtration and dried in vacuoovernight. There was 100% substitution by nmr analysis.

2. Preparation of HO—PEC_(7K)—OH

HO—(—CH₂CH₂—O—)_(n)—H (n_(ave)=23) (Aldrich, 10 g) was dissolved in 50ml of anhydrous acetonitrile and the solvent removed by distillation.The residue was dissolved in 20 ml of anhydrous acetonitrile and 10 g(1.0 equiv) of Bt-O₂C—(—O—CH₂CH₂—)_(n) OCO₂-Bt (n_(ave)=23)- and 2.5 gof dimethyaminopyridine were added. The reaction was stirred at 70° C.for about 22 hours. The product was precipitated with 500 ml ofisopropanol, stirred for half hour, then collected by vacuum filtration.The collected product was washed with isopropanol 200 ml×2), and driedin vacuo overnight. The resulting polymer was about 97% pure by gpc andnmr and had a molecular weight of 7000 Da.

EXAMPLE 3

Synthesis of bis-benzotriazolyl carbonate of HO-PEC_(7KDa)—OH

15.0 g of HO-PEC_(7KDa)—OH (prepared in Example 2) was dissolved in 50ml of anhydrous acetonitrile, and solvent was distilled off. Thisprocess was repeated once. The residue was dissolved in 50 ml ofanhydrous acetonitrile, and 1.8 g of bis(1-benzotriazolyl carbonate) and0.5 ml of anhydrous pyridine were added. The reaction was stirred atroom temperature under Ar overnight. The solvent was evaporated todryness at reduced pressure and the product was precipitated by additionof 500 ml of isopropanol. The precipitate was stirred under Ar for 30min., collected by vacuum filtration, and dried in vacuo overnight.Substitution was greater than 95% by nmr. Yield: 14.3 g.

EXAMPLE 4

Preparation of HO-[—(CH₂CH₂O—)_(n)CO₂]_(m)(—CH₂CH₂O)_(n)—H (n_(ave)=45;m_(ave)=18 from HO(—CH₂CH₂O—)_(n)CO₂NS (n_(ave)=45)(NS═N-succinimidyl)in solution

HO(—CH₂CH₂O—)_(n)CO₂NS (n_(ave)=45)(5 g) was dissolved in acetonitrile(10 ml), dimethylaminopyridine (0.6 g) was added, and the resultingmixture was stirred at 70-80° C. overnight. Methylene chloride (20 ml)was added and the resulting mixture added to 600 ml of isopropylalcohol. The resulting precipitate was collected by vacuum filtrationand dried under vacuum overnight at room temperature. Nmr and gpcanalysis indicated the polymer to have a molecular weight of 38,000 Da.

EXAMPLE 5

Preparation of HO-PEC_(35Kda)—OH from HO(—CH₂CH₂O—)_(n)CO₂NS(n_(ave)=45) (NS═N-succinimidyl) in a melt

HO(—CH₂CH₂O—)_(n)CO₂NS (n_(ave)=45)(5 g) and dimethylaminopyridine (0.6g) were mixed and the resulting mixture was stirred at 80-90° C.overnight. Methylene chloride (20 ml) was added and the resultingmixture added to 600 ml of isopropyl alcohol. The resulting precipitatewas collected by vacuum filtration and dried under vacuum overnight atroom temperature. Nmr and gpc analysis indicated the polymer to have amolecular weight of 35,000 Da.

EXAMPLE 6

Preparation of HO-PEC_(9.4KDa)—OH from HO—(—CH₂CH₂—O—)_(n)—H(n_(ave)=14) and di-(N-succinimidyl) carbonate

HO—(—CH₂CH₂—O—)_(n)—H (n_(ave)=14) (10.0 g) was dissolved in 100 ml ofacetonitrile and the solution was dried by azeotropic distillation.After removal of acetonitrile, 80 ml of methylene chloride and 20 ml ofacetonitrile, 2.7 ml of pyridine, and 4.3 g of di-(N-succinimidyl)carbonate were added. After stirring overnight, the solvent was removedunder reduced pressure and 10 ml of acetonitrile and 4 g of DMAP wereadded. The mixture was stirred at 80-90° C. for 5 h and the productprecipitated by addition of isopropanol. The product was collected byvacuum filtration and dried under vacuum at room temperature to obtainthe product as a white powder (4.5 g). NMR and GPC analysis indicatedthat the polymer has a molecular weight of 9400 Da.

EXAMPLE 7

Preparation of hydrogels from Bt-O₂C—O-PEC_(7KDa)—O—CO₂-Bt and 8-arm-PEGamine (10 kDa)

In a test tube, 110 mg of Bt-O₂C—O-PEC_(7KDa)—O—CO₂-Bt was dissolved in1 ml of phosphate buffer (0.1 M, pH 7.0). To it was added 0.36 ml of8-arm-PEG-amine (110 mg/ml in the buffer). After rapid shaking, it wasallowed to sit. A hydrogel formed within 2 hours.

EXAMPLE 8

Degradation of the hydrogel (Example 7) prepared from PEC and 8-arm-PEGamine (10 kDa).

The hydrogel (0.5 g.) was placed in 3 ml of PBS buffer or rat serum at37° C. Rat serum was replaced with fresh serum every 10 to 12 hours. Geldisappearance was monitored visually. The gel prepared from PEC_(7KDa)disappeared in PBS buffer within 3.5 days but in 1.5 days in rat serum,while the similar gel prepared from PEC3Kda disappeared in PBS buffer inapproximately 10 days.

EXAMPLE 9

Preparation of PEC_(7KDa) diacrylate

HO-PEC_(7KDa)—OH (10 g)(prepared in Example 2) in toluene (200 ml) wasdried by azeotropic distillation and triethylamine (1.75 ml) in CH₂Cl₂was added. The resulting solution was cooled to 0° C. and acryloylchloride (2.5 ml) was added under argon. After the addition wascomplete, the mixture was stirred overnight at room temperature. Theresulting solution was concentrated, filtered, and the productprecipitated from the filtrate by addition of ethyl ether (300 ml). Theprecipitate was collected by filtration, dissolved in 100 ml ofmethylene chloride, and the resulting solution stirred with 10 g ofsodium carbonate overnight. The mixture was filtered and the product wasprecipitated with 500 ml of ethyl ether. The product was collected byfiltration and dried under vacuum. NMR comparison of acrylate andbackbone absorptions indicated approximately 100% of the terminal groupswere acryloylated.

EXAMPLE 10

Preparation of hydrogel from PEC_(7KDa) diacrylate

PEC_(7KDa) diacrylate (100 mg) was dissolved in 1 ml of pH 7 buffer and30 ul of potassium persulfate was added. To the resulting solution wasadded 30 μl of 100 mM iron(II) sulfate solution. The solution was shakenand a hydrogel formed in several minutes.

EXAMPLE 11

Preparation of degradable hydrogel from NS—O₂C—O-PEC_(7KDa)—O—CO₂—NS andchitosan

NS—O₂C—O-PEC_(7KDa)—O—CO₂—NS (300 mg) was mixed with 3 ml of a 1 wt. %solution of chitosan at pH 4. The resulting solution formed a clear,firm hydrogel within 2 hours.

EXAMPLE 12

Preparation of HO-[—(CH₂CH₂O—)_(n)CO₂]_(m)(—CH₂CH₂O)_(n)—H (N_(ave)=22;m=2 from benzyl-O—(CH₂CH₂O)_(n)—H (n_(ave)=22) andBt-O₂C—O—(CH₂CH₂O)_(n)—CO₂-Bt (n_(ave)=22)

Azeotropically dried benzyl-PEG_(1KDa)—OH (40.6 g, 40.6 mmoles),Bt-O₂C—O—PEG_(1KDa)—O—CO₂-BT (25 g, 20.3 mmoles) anddimethylaminopyridine (5 g, 81.2 mmoles) was mixed in 300 ml ofacetonitrile. The reaction was stirred at 75-80° C. overnight. Thesolvent was removed by rotary evaporation and the product wasprecipitated using diethyl ether/isopropyl alcohol (500 ml, 50/50) in anice bath. The product was collected by filtration and dried undervacuum. The dry product was placed in a 500 ml Parr bottle and dissolvedin 100 ml of anhydrous 1,4-dioxane. 11.0 g of Pd/C (10% pd by wt.) wasadded and the bottle pressurized with 45 psi of hydrogen and shaken for20.5 hours. The mixture was filtered to remove the Pd/C and the solventremoved by rotary evaporation. The crude solid was precipitated byadding 500 ml of isopropyl alcohol. The product was collected byfiltration and dried under vacuum. The product was demonstrated to begreater than 98% pure with a polydispersity of 1.034 by GPC.

Preparation of NS-0₂C—O-PEC_(3KDa)—O—CO₂—NS(NS═N-succinimidyl) fromHO-PEC_(3KDa)—OH

The dried HO-PEC_(3KDa)—OH was dissolved in anhydrous CH₃CN (150 ml) andstirred at room temperature under argon. Di-N-succinimidyl carbonate(2.6 g, 10 mmoles) was added and allowed to dissolve before pyridine(0.81 ml, 10 mmoles) was added. The reaction mixture was stirredovernight (15 hrs.) The CH₃CN was removed under vacuum. The crude solidwas dissolved in CH₂Cl₂ (100 ml) and washed once with buffer solution(100 ml). The aqueous layer was extracted with an additional 50 ml ofCH₂Cl₂ and the combined organic layers were dried using Na₂SO₄. Thesolvent was removed under vacuum until a viscous residue remained. Tothis was added IPA (500 ml) to precipitate the product. The product wascollected by filtration and dried under vacuum. The product wasdemonstrated by ¹H nmr to have greater than 95% purity.

EXAMPLE 13

Hydrolysis Kinetics of a Single Carbonate Linkage in a PEC Polymer

CH₃O—(CH₂CH₂O)_(n)—CO₂(CH₂CH₂O)_(n)—OCH₃ (n_(ave)=113) (1 wt. % inbuffer, pH 7.4) was thermostated at 37° C. and the hydrolysis rate ofthe carbonate linkage was measured by analysis of the product using GPC.The half-life for hydrolysis under these conditions was found to be 44days.

The foregoing description is to be considered illustrative rather thandescriptive of the invention. Therefore, it should be understood thatthe specific embodiments described herein are illustrative of how theinvention may be practiced and that modifications and other embodimentsare intended to be included within the scope of the appended claims.

1. A polymer having the formula of:X—O—{[(R₁—O)_(a)—CO₂—]_(h)—[(R₂—O)_(b)—CO₂]_(i)}_(m)—{[(R₃—O)_(c)—CO₂—]_(j)—[(R₄—O)_(d)—CO₂—]_(k)}—(R₅—O)_(c)—Ywherein R₁, R₂, R₃, R₄, and R₅ are methyl groups; a, b, c, d, and e areintegers of from about 5 to 500; h, i, j, and k are integers of from 0to about 100, and the sum of h, i, j, and k is from about 5 to 100; andX and Y are independently selected from the group consisting ofhydrogen, alkyl, alkenyl, aryl, and reactive moieties.